The role of presynaptic terminals in the initiation and maintanance of seizure discharge will be investigated, principally by using the spinal dorsal root ganglion (DRG) cell as a test object. DRG cell bodies are known to share the chemical sensitivities of the intraspinal terminals of their axons. The effect of penicillin and other convulsant drugs will be investigated on the following properties of DRG cells: electrical excitability; nonlinearities in voltage-current functions; sensitivity to gamma-amino butyric acid; chloride permeability in the resting state, and under the influence of GABA. Where possible, the tests on DRG cells will also be carried out on intraspinal afferent terminals. The role of ionized calcium in maintaining excitability levels, and in the generation of seizures will be investigated. Ion-selective microelectrodes will be used to measure calcium activity near neurons. Local calcium activity will be caused to vary above and below the "resting" level and will be related to neural excitability. The exchange of calcium between blood and brain under conditions of hyper- and hypocalcemia will be studied. Such exchanges will be observed under conditions of normal hormone regulation, and also after the administration of parathormone and of calcitonin. The critical level of brain (Ca2 ion)o for the triggering of seizure discharges will be determined.